1. Field of the Invention
The subject invention is directed to fuel injection, and more particularly, to systems and methods for cooling the exit ports of a main fuel circuit of a staged airblast fuel injector using the pilot fuel flow, at low engine power.
2. Background of the Invention
Staged fuel injectors for gas turbine engines are well known in the art. They typically include a pilot fuel atomizer for use during engine ignition and low power engine operation and at least one main fuel atomizer for use during high power engine operation in concert with the pilot fuel atomizer. One difficulty associated with operating a staged fuel injector is that when the pilot fuel circuit is operating alone during low power operation, stagnant fuel located within the main fuel circuit can be susceptible to carbon formation or coking due to the temperatures associated with the operating environment. This can degrade engine performance over time.
In the past, attempts were made to passively insulate or otherwise protect the main fuel circuit of a staged fuel injector from carbon formation during low power engine operation using heat shields or vents. More recently, efforts have also been made to actively cool a staged fuel injector using the fuel flow from the pilot fuel circuit. U.S. Pat. No. 7,506,510, which was issued to the present inventor and is herein incorporated by reference in its entirety, discloses the use of active cooling to protect against carbon formation in the main fuel circuit of a staged airblast fuel injector. A partial cross-section of the fuel injector nozzle disclosed in U.S. Pat. No. 7,506,510 is shown in FIG. 1. The disclosed staged fuel injector includes a main fuel circuit (170 outer, 172 inner) which delivers fuel to a main fuel atomizer and a pilot fuel circuit (160 outer, 162 inner) for delivering fuel to a pilot fuel atomizer (not shown) located radially inward of the main fuel atomizer. Portions of main fuel and pilot fuel circuits are formed in prefilmer 124 and fuel swirler 126. A spin chamber 128 is formed between the downstream end of the prefilmer 124 and the fuel swirler 126. The main fuel circuit 170/172 delivers fuel to the spin chamber 128 through axially arranged exit slots 170d formed in the fuel swirler 126. The pilot fuel circuit 160/162 is in close proximity to the main fuel circuit 170/172 enroute to the pilot fuel atomizer, so that the pilot fuel flow cools stagnant fuel located within the main fuel circuit 170/172 during low engine power operation to prevent coking.
However, as shown in FIG. 1, the staged fuel injector described in U.S. Pat. No. 7,506,510 uses a single cooling channel 160 to cool the main atomizer exit slots 170d during low power operation. As the staging requirements of lean burn combustion have matured, the need for higher power staging points has increased. The cooling design described in U.S. Pat. No. 7,506,510 functions well at lower compressor discharge temperatures, like those up to about 30% maximum take-off thrust, but in the recent staging requirements the cooling channels must be capable of performing at engine power levels of up to 60% of the maximum take-off thrust. This represents a substantial increase in the compressor discharge temperature of the air and overheats the stagnant fuel in the un-staged main atomizer. Therefore, additionally cooling is necessary.
Accordingly, there is a need in the art for an improved method of actively cooling a staged piloted air blast or dual prefilming pure airblast fuel injector to prevent carbon formation or coking in the main fuel circuit during low power engine operation and in general, to enable the pilot fuel flow to cool the main fuel circuit during high power engine operation, so as to enhance the engine performance and injector life.